University of Minnesota
Physicists Keith Goetz (left) and John Wygant are part of NASA's Solar Probe Plus mission, which will send a spacecraft closer to the sun than any has gone before.
Photo by Patrick O'Leary
The sun in close-up
A U of M instrument will fly on a NASA spacecraft right into the sun’s corona
By Deane Morrison
Swooping closer to the sun than any previous mission, a new NASA spacecraft will probe the star's corona—and its two biggest secrets—with help from University of Minnesota physicists.
The physicists, along with colleagues at other institutions, comprise one of four teams chosen to design experiments for the Solar Probe Plus (SPP) mission.
The size of a small car, the SPP spacecraft will perform seven flybys of Venus, tapping the planet's gravity to steer itself ever nearer to the sun. At its closest, the vehicle will sail through the corona (the sun's plasma "atmosphere") just four million miles from the sun's core. That's one-eighth the average distance of piping-hot Mercury, the sun's nearest planetary neighbor.
With a front row seat like that, SPP is sure to discover a wealth of new and unexpected behaviors from our parent star.
A sizzling slice of space
At the distance of SPP's closest approach, 4 million miles (9.5 solar radii), the sun would appear 23 times wider, cover more than 500 times as much sky, and shine 500 times brighter than it does as seen from Earth. And unlike mythical Icarus's wings, SPP's carbon-ceramic heat shield will withstand temperatures of 1,400 C (2,550 F).
"Your car would turn into a molten puddle at that temperature," says Keith Goetz.
"It's a dream mission," says physicist Keith Goetz, principal investigator for the University's SPP team. "It's the most interesting thing in space physics."
Besides answering scientific questions, the mission, scheduled for launch in 2018, will help researchers characterize and predict how shock waves in the corona energize particles that could pose a danger to future space explorers.
Fields and streams
The experiments aboard the SPP spacecraft will address two mysteries that have fascinated physicists for decades.
First, it seems logical that the temperature would drop as one moves away from the sun; instead, it rises sharply. The sun's surface registers a relatively cool 6,000 degrees C, but at a blistering one million C, the inner corona is hundreds of times hotter. Thus, some mechanism must exist to transfer energy to the corona, where it is dissipated as heat.
"Whatever heats the corona has to be electric and magnetic fields. That's all there is," notes Goetz. "There are competing ideas about the details, but no measurements or observations." Data from SPP should, finally, settle the debate.
The second enigma centers on the solar wind, a 3-D stream of charged particles flowing out from the sun at speeds in excess of 400 kilometers per second—a whopping million miles per hour. A dynamo in the corona must accelerate the particles, but its identity is unknown. Again, researchers believe electric and magnetic fields are involved.
The sun's corona becomes spectacularly visible during a total solar eclipse. See second sidebar.
"SPP lets us finally make the measurements we need to understand why the solar wind blows the way it does," Goetz says.
One smart instrument
The U of M's share of those measurements will come from an instrument called the time domain sampler, or TDS, which Goetz designed.
"It takes snapshots of interesting short-term things," he says.
He likens it to "a souped-up car radio," a somewhat understated term for a device that measures electric and magnetic fields, radio waves, and the abundances and sizes of interplanetary dust particles. It's also smart enough to periodically sift through its stored data, then select and save only what promises to be useful or intriguing.
The corona (from the Latin word for "crown") is the upper, ionized atmosphere of the sun. But the part we see during an eclipse is just the tip of the iceberg; the corona actually extends much farther out into space.
The TDS experiment is part of Fields, one of the four sets of projects chosen by NASA for SPP. Also on the U of M Fields team are physics professors Cynthia Cattell and John Wygant, who will analyze the data, and retired physics professor Paul Kellogg. The principal investigator for Fields is Stuart Bale, who received bachelor's and doctoral degrees in physics from the University of Minnesota and now heads the Space Sciences Laboratory at the University of California, Berkeley.
All are excited by the prospect of discovering more about how electric and magnetic fields interact with particles to produce large-scale effects.
"This work has implications for space weather, other stars, black holes, and so on," says Goetz. 'They're all affected by the same physical laws."
Published in 2010